docs: def call_cc(fn: Callable) -> 'Observable':
          r"""call-with-current-continuation.
  
          Haskell: callCC f = Cont $ \c -> runCont (f (\a -> Cont $ \_ -> c a )) c
          """
          def subscribe(on_next):
              return fn(lambda a: Observable(lambda _: on_next(a))).subscribe(on_next)
  
          return Observable(subscribe)